Problem: Which of these numbers is prime? ${13,\ 49,\ 52,\ 75,\ 93}$
A prime number has exactly two factors: 1 and itself. The factors of 13 are 1 and 13. The factors of 49 are 1, 7, and 49. The factors of 52 are 1, 2, 4, 13, 26, and 52. The factors of 75 are 1, 3, 5, 15, 25, and 75. The factors of 93 are 1, 3, 31, and 93. Thus, 13 is a prime number.